Can't change horizon color: "can't edit driven number value, see graph editor for the drive setup"
The purple border indicates that there is a Driver present for that value. A Driver is a way of programatically controlling a value - similar to keyframing. When a value is being Driven you cannot manually change it. Move the mouse to over the field and press Alt-D to remove it (or right-click for the menu which also lets you add/remove drivers). You probably pressed D at some point without realising whilst hovering over the field and this would have added the driver.